1. Field of the Invention
This invention relates to noise reduction in reading out data from radiation detecting elements and more particularly to a process and related apparatus for reducing noise primarily due to addressing and switching circuitry in a radiation detecting panel comprising a plurality of detecting elements.
2. Description of Related Art
Capturing radiation and obtaining an electronic signal representative of the intensity of the radiation directly, is well known. The development, relatively recently, of large scale integration and thin film transistor technology, has made it possible to make direct radiation capture elements, in which the radiation intensity is captured in minute detectors and stored in the form of an electrical charge, which is subsequently read-out and converted to an electronic signal. A plurality of such may be used to form a detection panel, wherein the individual detection element signals may be assembled into a composite signal which may be displayed to produce a visible image corresponding to a modulated radiation intensity pattern.
A detailed description of at least one such radiation detecting element for detecting X-ray radiation, and a radiation detecting panel comprising a plurality of such elements is given in a chapter by Denny L. Lee, Lothat S. Jeromin, and Lawrence K. Cheung entitled "The physics of a new Direct Digital X-ray Detector" in a book by Springer Verlac, entitled Computer Assisted Radiology, published in 1995 in Berlin, and in U.S. Pat. Nos. 5,319,206 and 5,331,179, issued to Lee et al. on Jun. 7, and Jul. 19, 1994 respectively.
Radiation capture elements of the type described in the above references, typically include a radiation sensor which generates a charge proportional in magnitude to the incident radiation, and include means for storing and reading out the charge. The read-out means again typically provide switching and multiplexing circuitry which is used to transfer the generated charges from the capturing element to a charge conversion circuit where these charges produce an electrical radiation data signal which may be further manipulated as needed to generate a visible image.
Typical prior art read-out circuitry comprises an integrating amplifier connected to the element charge storage means which is often a capacitor, through a switch which is typically an FET transistor accessible from outside the element, to permit turning it on and off at will. The amplifier also includes an enable/reset switch which permits to discharge any charges stored in the integrating circuit prior to receiving and integrating the charges from the element following exposure to radiation.
A problem with integrating amplifiers and enable/reset switches is that the switching action itself to enable the amplifier following resetting, generates a noise signal which is integrated in the amplifier and which is added to the subsequent radiation data signal generated by the integration of the stored charges in the charge storage means in the detecting element. We shall refer to this noise signal as the enable/reset noise. Thus the output of the integrating amplifier contains both the radiation data signal from the element and the noise signal from the enable/reset switch, the latter being an undesirable component. Addressing and switching the switch or switches in an element, or in a plurality of elements, as the case may be, to transfer the charge from the detection element to the integrating amplifiers, is usually done by the application of a trigger voltage pulse to the gate of an FET transistor forming the switch. This activity results in a second type of noise from a combination of sources, some relating to the panel structure, some to each switch structure, and some to differences in the exact timing structure of the line addressing gate pulses which are not absolutely identical in each occurrence.
We refer to these row related noise signals collectively as the common mode row noise signal or in short the common mode noise. This common mode noise is a second undesirable component in the output of the integrating amplifier, which needs elimination in order to obtain a noise free radiation exposure data signal from the detecting element.
It is also well known in the art to enhance radiogram images to improve diagnostic performance by eliminating noise. However, this has generally been done by analyzing the entire data set of an image, or of a desired image region. Typically this is done by taking a histogram of the image data, including noise, and adjusting the gradation processing conditions so that the portions corresponding to 5% and 95% are assigned minimum and maximum output signal values, respectively.
Other attempts to minimize noise include segmenting the desired region in a digital radiographic image from foreground regions which have received very little radiation due to the use of radiation limiting elements, for example collimator blades used to define the size and shape of the radiation field or pattern and from background regions which are regions of direct radiation exposure around the body part being examined. Image data in the radiographic image are identified using edge detection techniques and subsequently assigned to a block which is empirically defined as foreground/background or object. In subsequent image enhancement calculations, foreground/background blocks are not included in the calculation of enhancement parameters. This process is calculation intensive and is practiced on an overall digital radiographic image basis but does not account for common mode noise, or enable/reset switch noise.
There is thus still need for circuitry, systems, and read-out methods which can effectively isolate and eliminate such enable switch and common mode noise components in the radiation data signal, obtained from radiation detecting elements which convert incident radiation to stored electrical charges and signals.